The priority document for this application, Japanese Patent Application No. 11-221995, is herein incorporated by reference in its entirety, all essential material having been set forth in the specification.
1. Field of the Invention
The present invention relates to an impedance measuring apparatus for an electronic component using a four-terminal method (Kelvin method).
To simplify the description in this specification, an example has been chosen involving the measurement of a resistance to which DC signals are applied. Due to the natures of DC, the only parasitic parameters generated in a circuit are resistance components. When measurement is applied in the case of AC signals, though the parasitic parameters become impedances which are denoted using complex numbers, the concepts are akin to those for the measurement using DC signals.
2. Description of the Related Art
Hitherto, a two-terminal method, as shown in FIG. 1, is used to measure the impedance of an electronic component. In this case, a measured value of the impedance can be measured as RM=V/I. RM, which is measured using this method, includes contact resistances RH and RL occurring in measurement cables (or terminals), other than the impedance Rdut of the electronic component, which is a measurement object. This contact resistance includes lead-wire resistances of the measurement cables or the like. RM may be expressed as:
RM=V/I=Rdut+RH+RL
When RH and RL are very high relative to Rdut, making the value of Rdut negligible, a measurement error results. The contact resistances RH and RL vary whenever contact occurs between the measurement object and the measurement terminals. Accordingly, the influence of RH and RL cannot be removed by means of compensation or the like.
When the measurement error caused by the contact resistances RH and RL is not negligible, such as a case in which the measurement object has a low impedance, the measurement may be performed using a four-terminal method, as shown in FIG. 2. In this method, the measured value RM=V/I=Rdut is obtained, and RH and RL are avoided as measurement error factors.
However, there are problems in the four-terminal method when contact failure occurs at voltage detection lines (Hp, Lp). For example, when many measurement objects are measured one after another, the Hp line is subject to contact failure, as shown in FIG. 3. If stray capacitance CHP is generated on the Hp line at this time, the stray capacitance appears to have been charged by the voltage obtained on the measurement object before the present measurement of current was obtained. When a measurement object is measured in this state, the following expression is obtained.
RM=V(previous voltage)/I(present current)=Rdutxe2x80x83xe2x80x83(1)
The value obtained in this manner is not the resistance of the measurement object currently intended to be measured, as it is influenced by the most recent normal measurement of the measurement object. There is a possibility that measurement failure may occur on the Lp line as well as the Hp line, for similar reasons.
Therefore, when a contact failure occurs on a voltage detection line, the measured resistance value is not accurate when a xe2x80x9cpurexe2x80x9d four-terminal method is used. Because such inaccuracies, there is a risk of delivering defective products instead of good products. In regard to current measurement, when a contact failure occurs at a current-carrying line Hc or Lc, measurement cannot be performed since the current I becomes zero.
It is desirable to use a voltage detection unit having a high input impedances RINH and RINL in the measuring apparatus, yet the input impedances are not infinite. In addition, since the impedance of stray capacitance of the measurement cable is inserted so as to be in parallel, the input impedance is decreased. Accordingly, the voltage detected at the voltage detection unit is voltage-divided by the contact resistances RHP and RLP, and RINH and RINL. When RHP and RLP become too high to be negligible, a measurement error occurs. Since RHP and RLP vary when contact occurs between the measurement object and the measurement cables, the measurement error due to this cannot be removed by means of a method such as compensation. Furthermore, when measurement using AC signals is performed, there is a possibility that measurement failure may occur because of electrostatic coupling or electromagnetically inductive coupling among the Hc and Lc lines, and the Hp and Lp lines.
Accordingly, it is an object of the present invention to provide an impedance measuring apparatus for an electronic component in which use of a simple circuit prevents a defective product from being inadvertently determined as a good product when contact failure occurs at a measurement terminal thereof.
It is another object of the present invention to provide an impedance measuring apparatus for an electronic component which decreases the measurement error due to contact resistance.
To this end, according to a first aspect of the present invention, there is provided an impedance measuring apparatus for an electronic component that measures the impedance of the electronic component using a four-terminal method. The impedance measuring apparatus for the electronic component includes a first current-carrying line and a first voltage detection line connected to one electrode of the electronic component; a first resistor establishing a connection between the first current-carrying line and the first voltage detection line; a second current-carrying line and a second voltage detection line connected to the other electrode of the electronic component; and a second resistor establishing a connection between the second current-carrying line and the second voltage detection line. In the impedance measuring apparatus, the first resistor and the second resistor have sufficiently high resistances compared to contact resistances occurring among the electrodes of the electronic component, the current-carrying lines, and the voltage detection lines.
According to a second aspect of the present invention, there is provided an impedance measuring apparatus for an electronic component for measuring the impedance of the electronic component using a four-terminal method. The impedance measuring apparatus for the electronic component includes a first current-carrying line connected to one electrode of the electronic component and a first voltage detection line connected to the other electrode thereof; a first resistor establishing a connection between the first current-carrying line and the first voltage detection line; a second current-carrying line connected to the other electrode of the electronic component and a second voltage detection line connected to the one electrode thereof; and a second resistor establishing the second current-carrying line and the second voltage detection line. In the impedance measuring apparatus, the first resistor and the second resistor have sufficiently high resistances compared to contact resistances occurring among the electrodes of the electronic component, the current-carrying lines, and the voltage detection lines.
The impedance measuring apparatus according to the first aspect of the present invention may be used to effectively determine whether a product is defective or not when the impedance thereof is higher than a standard value. That is, in a measuring apparatus according to the first aspect of the present invention, when the measuring terminals and the electronic component are in good contact with each other, the result is a measurement equivalent to the four-terminal method, and the measured value is the impedance of the electronic component. If any of the measuring terminals fails to be in contact with the electronic component, the result is that the measurement method is converted from the equivalent of a four-terminal method to a two-terminal method at an electrode unit in which contact failure occurs. When contact failure occurs, the measured value is increased by the corresponding amount of the contact resistances. By pulling-up the voltage detection lines, contact failure may be estimated, to reduce or eliminate the risk of delivering defective products as good products.
When contact failure occurs, it is desired to estimate the location of the contact failure. In accordance with a further embodiment of the present invention, a third resistor may be connected to the current-carrying line provided between the one electrode of the electronic component and the first resistor; and a fourth resistor may be connected to the current-carrying line provided between the other electrode of the electronic component and the second resistor. Preferably, the resistances of the third and fourth resistors are sufficiently lower than the first and second resistors, and are sufficiently higher than the contact resistances, with the resistors each having a different resistance value. In this case, by changing the resistances of the third resistor and the fourth resistor, it is possible to estimate at which voltage terminal contact failure occurs.
In addition to connecting resistors to the current-carrying lines, a fifth resistor and a sixth resistor may be connected to the voltage detection lines. By setting the resistances of the third to the sixth resistors so as to be different from one another, it is possible to estimate every location of the occurrence of contact failure.
The impedance measuring apparatus according to the second aspect of the present invention may be used to effectively determine whether a product is defective or not when the impedance thereof is lower than a standard value. That is, in a measuring apparatus according to the second aspect of the present invention, when contact failure occurs on any of the measuring terminals, the measured value is decreased by an amount corresponding to the contact resistances. By pulling-down the voltage detection lines, contact failure may be estimated, to reduce or eliminate the risk of delivering defective products as good products.
In the impedance measuring apparatus according to the first or the second aspect of the present invention, it is desirable to have a sufficiently high input impedance of a voltage detection unit of the measuring apparatus. When measurement is performed using AC signals by means of the four-terminal method, there is a case in which a measuring apparatus has a high input impedance using a DC signal, yet has low input impedance for an AC signal due to input capacitance. In addition, even though the input impedance of the measuring apparatus is high, the input impedance thereof is lowered due to stray capacitance of a measurement cable. In these cases, preferably, by inserting a voltage follower having a high input impedance ahead of an input unit of the measuring apparatus, the influence due to contact failure is lessened, which can decrease the measurement error.
The voltage follower is preferably provided on at least the higher-voltage detection side of the voltage detection lines. It may be provided on the lower-voltage detection side of the voltage detection lines when necessary.
In addition, the impedance measurement can be performed by inserting resistors in the measuring circuit. Since no particular contact detection circuit is required, the measuring apparatus in accordance with the present invention may advantageously be realized with low cost.